It is known to add various additives to an oil of lubricating viscosity to lubricate a marine diesel engine to reduce wear and improve cleanliness. Typically the marine diesel cylinder lubricant is metered into an engine at 1.2 to 1.3 g/kW hr, that is, 1.2 to 1.3 g/hr per kW of engine power. Due to environmental concerns over emissions of SOx (sulphur oxides), NOx (nitrogen oxides) and particulate matter such as soot and oxides of sulphur from fuels and a desire to reduce operational costs, it would be desirable to lubricate a marine diesel engine with a lower lubricant treat rate. Lowering the sulphur in fuel, may result in a reduction in emissions of SOx and/or NOx; and allows for the lowering of lubricant treat rate. However, simply reducing the feed rate of the lubricant also reduces the treat rate of other additives, including antiwear agents and deposit control additives. As a consequence lowering the lubricant treat rate would result in the engine receiving at least one of reduced amounts of detergent (which leads to reduced amounts of TBN), reduced antiwear additives and reduced deposit control additives. Decreasing the amounts of TBN is likely to cause an increase in the amount of un-neutralised acids in the lubricant. As the amount of un-neutralised acid accumulates, this results in increased engine wear (including corrosive, abrasive or adhesive wear), increased deposit formation, and decreased cleanliness.
U.S. Pat. No. 6,551,965 discloses a marine diesel engine lubricant with improved temperature performance. The lubricant contains overbased sulphurised phenate and sulphonate detergents an ashless dispersant and a zinc diaryldithiophosphate.
The present invention provides a process for allowing for a reduction in lubricant treat rates in a cost effective manner and without the adverse effects discussed above.